Newcombe, Thigmotropism of terresh-ial roots. 81 



species of seedlings whose behavior was described alxjve yiivc3 no 

 cnrves except one negative. 



Tlie area of root-sui'face sensitive to a rlieotropic Stimulus 

 has been determined^) for Zea mays (popcorn) and for Bhapha- 

 nus satkms to extend from the apex to a distance 20 mm from 

 the apex. Wlien 25 mm of the apex were shielded from tlie 

 water-stream by glass-tubes, tlie curves were so few as to make 

 it probable tbat the limit of sensitive tissue had been reached. 

 The third plant used in the present tests, Lupinus albus, had 

 not had its sensory area determined. This determination was 

 now made by coveriug the apical 25 mm of 18 roots with glass- 

 tubes, allowing the more proximal part of the root to be ex- 

 posed to a stream of water of favorable velocity in a basin. The 

 experiment continued for 31 hours, in a water temperature of 

 23 ° C. One root became positive, one negative, and 16 grew 

 straight. This result proves Lupinus not to be sensitive to a 

 water-stream back of 25 mm of the apex of the root, and shows 

 that the roots in the preceding experiment, covered with collo- 

 dion tubes of 25 mm in length as well as those wholly covered, 

 owed their curves to the pressui'e of water through the collo- 

 dion membrane. 



C. Summary and Conclusions. 



The foregoing pages are a record of work done in testing 

 the apical millimeter of the root of terrestrial plants, and the 

 part back of the apex, for sensitiveness to contact and pres- 

 sure. The experiments were made with seedlings , some of whicli 

 were used in damp-chambers at rest, some in damp-chambers re- 

 volved on klinostats, and some had their roots immersed in 

 water during the tests. Altogether 15 species of plants were 

 used, and several species were employed in each fest. 



Many roots in damp-chambers with little Squares of card- 

 board or paper attached to the side of the apical niilUmeter 

 bent away from the attached object. Bits of baked or unbaked 

 clay or plaster of Paris also caused negative curves, though not 

 so many as the paper. To such bodies as small scales of giass 

 and of mica attached to the side of the apical millimeter, the 

 roots were generally indifferent. It appearing that results ob- 

 tained from experiments like the foregoing might be unreHable 

 because of the possible hydrotropic behavior of i\\Q roots when 

 objects were applied to the a])ex in the damp-chamber, a better 

 series of tests was made with the roots immersed in water. 

 In some of the experiments of this series the roots were allow- 

 ed to grow down against curved objects such as the sides of 

 crystallizing dishes, the notion being that if the tip were sensi- 

 tive it might follow the curving surface instead of obeying the 

 influence of gravitation. No certain responses were obtained by 

 this method. Next, the tips of the roots were brought against 



1) Newcombe, The sensory zoiie of roots. (Anu. of Bot. XVI. 431.) 

 Beihefte Bot. Centralbl. Bd. XVII. 1904 6 



